Genetic Counselling and Prenatal Diagnosis
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Direct answer
Genetic counselling and prenatal diagnosis aim to estimate a couple's recurrence risk and detect a fetal disorder early enough to inform decisions. Screening tests such as the nuchal translucency scan and the triple test give a probability of disease, while chorionic villus sampling and amniocentesis give a diagnosis. Because invasive tests carry a small risk of pregnancy loss, an abnormal screen is always followed by a diagnostic test.
What you must remember
- Screening versus diagnosis: screening estimates risk and cannot confirm a disorder, whereas CVS or amniocentesis gives a definitive karyotype or DNA result.
- NT scan: measured at 11 to 14 weeks, an increased nuchal translucency raises the risk of chromosomal anomalies such as Down syndrome.
- Triple test: maternal serum alpha-fetoprotein, unconjugated oestriol and free beta hCG are measured in the second trimester, and low AFP and oestriol with high hCG suggest Down syndrome.
- Quadruple test: it adds inhibin A to the triple test, which improves detection of Down syndrome.
- CVS: performed at about 10 to 13 weeks, it provides an early karyotype or DNA result from placental tissue.
- Amniocentesis: performed after about 15 weeks, it gives a karyotype and measures amniotic fluid AFP for neural tube defects.
- Cell-free fetal DNA: a maternal blood test from about 10 weeks, it is a screening test, so a positive result needs confirmation.
- PCPNDT Act: it prohibits prenatal sex determination and disclosure of the fetal sex, so counsellors must decline.
Matching the test to the question
Each prenatal test answers a different question, and gestation decides which tests are possible. The table below lists the commonly used options in the order they are offered during pregnancy.
| Test | Type | Usual timing | Main purpose |
|---|---|---|---|
| NT scan | Screening | 11 to 14 weeks | Chromosomal risk from nuchal translucency |
| Triple or quadruple test | Screening | Second trimester | Down syndrome and neural tube risk |
| Cell-free fetal DNA | Screening | From about 10 weeks | Chromosomal risk from maternal blood |
| Chorionic villus sampling | Diagnostic | 10 to 13 weeks | Karyotype or DNA from placental tissue |
| Amniocentesis | Diagnostic | After about 15 weeks | Karyotype and amniotic fluid AFP |
Screening tests give a probability, so a high-risk result is followed by a diagnostic test before any decision is made. Diagnostic tests carry a small risk of miscarriage, which is why they are offered after a high-risk screen or a known familial risk.
Counselling begins with a family pedigree, from which the counsellor calculates the recurrence risk for each inheritance pattern. In autosomal recessive disease, two carrier parents have a 25 per cent risk for each pregnancy, and in autosomal dominant disease an affected parent has a 50 per cent risk.
Prenatal questions that catch students out
Most errors come from mixing up what a screen can prove with what a diagnostic test proves.
- Trap: calling the triple test diagnostic for Down syndrome. Correct: it is a screening test that gives a risk, so an abnormal result needs CVS or amniocentesis for karyotype confirmation.
- Trap: choosing amniocentesis at 10 weeks. Correct: CVS is the early diagnostic option at 10 to 13 weeks, while amniocentesis waits until after about 15 weeks.
- Trap: disclosing the fetal sex to parents on request. Correct: the PCPNDT Act prohibits prenatal sex determination and its communication, so counsellors must decline even when asked directly.
Frequently asked questions
Which screening test uses nuchal translucency to assess chromosomal risk?
The nuchal translucency scan measures fluid at the back of the fetal neck, usually at 11 to 14 weeks. An increased measurement raises the risk of chromosomal anomalies such as Down syndrome. Combining it with maternal serum markers improves detection, and abnormal results need diagnostic confirmation.
What is the difference between the triple test and the quadruple test?
The triple test measures maternal serum alpha-fetoprotein, unconjugated oestriol and free beta hCG in the second trimester. The quadruple test adds inhibin A to these three markers and improves detection of Down syndrome. Both are screening tests that give a risk estimate rather than a diagnosis.
Which diagnostic test is performed earliest in pregnancy?
Chorionic villus sampling is the earliest diagnostic prenatal test, usually performed at about 10 to 13 weeks. It gives a karyotype or DNA result from placental tissue, which allows decisions earlier than amniocentesis. It carries a small risk of pregnancy loss, so it is offered when the indication justifies it.
Why is cell-free fetal DNA testing considered a screening test?
Cell-free fetal DNA testing analyses fetal DNA circulating in maternal blood and estimates the risk of common chromosomal conditions. It is not diagnostic, so a high-risk result must be confirmed by chorionic villus sampling or amniocentesis. Counsellors explain this limit clearly before the test is taken.
Which Indian law restricts the disclosure of fetal sex?
The Pre-Conception and Pre-Natal Diagnostic Techniques Act prohibits prenatal sex determination and its communication to the family. Genetic counselling centres and clinicians must not reveal the sex of the fetus in any form. Breach of the Act attracts penalties, which makes this a common examination point.